1. Field of the Invention
The present invention relates to a piezoelectric device that includes a piezoelectric substance thin film bonded to a supporting substrate and a method for manufacturing the piezoelectric device.
2. Description of the Related Art
In recent years, many piezoelectric devices that include a piezoelectric substance thin film have been developed (see, for example, Japanese Unexamined Patent Application Publication No. 6-326553). In general, in piezoelectric devices that include a piezoelectric substance thin film, the piezoelectric substance thin film is bonded to a supporting substrate. Various methods for bonding a piezoelectric substance thin film to a supporting substrate have been proposed and used. For example, in a bonding method referred to as hydrophilic bonding (see Japanese Unexamined Patent Application Publication No. 6-326553), an inorganic oxide layer is formed on both the bonding surface of a mirror-finished piezoelectric thin film and the bonding surface of a supporting substrate. Hydroxy groups are then formed on the surface of the inorganic oxide layer. The surface of the inorganic oxide layer on the piezoelectric thin film side is then brought into contact with and is bonded to the surface of the inorganic oxide layer on the supporting substrate side through hydrogen bonding between the hydroxy groups. The bonding strength between the inorganic oxide layer on the piezoelectric thin film side and the inorganic oxide layer on the supporting substrate side is significantly improved by heat treatment at 200° C. or more for eliminating H2O from the hydrogen-bonded hydroxy groups.
In a bonding method referred to as activated bonding, the bonding surface on a mirror-finished piezoelectric thin film side and the bonding surface on a supporting substrate side are sputter-etched in an inert gas atmosphere or in a vacuum to remove contaminants from the surface and activate the surface. The bonding surface on the piezoelectric thin film side is then brought into contact with and is firmly bonded to the bonding surface on the supporting substrate side through an amorphous layer.
In a bonding method using a resin adhesive layer, the adhesive layer is formed on the bonding surface on the piezoelectric thin film side or on the supporting substrate side, and the bonding surface on the piezoelectric thin film side and the bonding surface on the supporting substrate side are then joined together and hardened to firmly bond the bonding surface on the piezoelectric thin film side to the bonding surface on the supporting substrate side.
In a bonding method that involves heat treatment at a relatively high temperature, such as 200° C. or more, as in hydrophilic bonding, a large difference in coefficient of linear expansion between the piezoelectric thin film and the supporting substrate results in a failure, such as separation of the piezoelectric thin film from the supporting substrate, due to thermal stress resulting from the heat treatment, and therefore it is impossible to stably achieve strong bonding.
On the other hand, unlike hydrophilic bonding, activated bonding can achieve strong bonding without high-temperature heat treatment. Thus, activated bonding is not substantially limited by the difference in coefficients of linear expansion between the piezoelectric substance and the supporting substrate.
However, activated bonding has a low resistance to surface contamination and requires strict management of the bonding environment. Thus, it is difficult to stably achieve strong bonding. Furthermore, it is impossible to achieve sufficient bonding strength with a certain combination of materials, for example, a LN substrate and a substrate having a silicon nitride film on the surface thereof.
In a bonding method using a resin adhesive, since the application of heat softens the adhesive, a difference in coefficients of linear expansion between the supporting substrate and the piezoelectric thin film may cause a misalignment between the supporting substrate and the piezoelectric thin film after heating, causing the surface to become undulated.